Lithium-ion (Li-ion) batteries tend to become dangerous when they are overcharged at high temperatures. Safely charging these batteries has become one of the most important design specifications in batterypowered portable equipment. Progress has been made in establishing industry standards such as the Japan Electronics and Information Technology Industries Association (JEITA) guidelines for improving battery-charging safety. However, this only applies if the battery temperature is exceeded before or around when the charging process began.
There are many instances where the battery could have been fully charged and then the battery temperature could have risen as it was taken into a hot car or the battery is exposed to heat coming from other peripherals. This concept can be extended for long-term storage of battery-enabled products, and certain requirements for shipping within a certain state of charge (SOC) comes into account. Similar principles discussed in this application report can be applied to design around supercapacitors as well.
This application example discusses how a designer can take this requirement into consideration and design a reliable and safe product. While this application note talks about warm and hot cases, it can be extended to other temperature requirements or actions that can be performed. This example uses the BQ25150 battery charger device, along with internal ADC comparators and a current sink, to slowly discharge the battery. Once a safe battery voltage is reached, the current sink is thereafter cut-off. An automatic fault checking routine is set to achieve very low power consumption and ensure that the bias currents for checking the battery pack thermistor is only turned ON when taking a measurement. This saves power and is less workload for the microcontroller since it does not always have to sample external channels.